1. Field of the Invention
The invention relates to an electrostatic lens system consisting of several electrodes and a method of making the same. More particularly, the underlying invention relates to a lithography apparatus where such a lens system is used in a highly advantageous manner.
2. Related Art
Electrostatic lenses consisting of electrodes at different potentials have to meet certain criteria in view of the roundness and surface quality as well as the relative adjustment of the lens electrodes with respect to each other in order to achieve the required image qualities. In a system having more than one lens, the relative adjustment of the lenses with respect to each other is of great importance too. Further, positioning errors of the particle source and its extracting system may deteriorate the image quality.
In a charged particle lithography system, charged particles originating from a suitable source are transmitted through a mask defining a specific pattern in order to transfer this pattern, with or without a scaling factor, to a substrate. The imaging as well as the mask illuminating system may be composed of one or several electrostatic lenses. For many purposes, it is important that the shape of the pattern on the substrate is to a high extent conformal to the pattern on the mask, i.e., to create an electrostatic column having negligible deviation of pattern placement from the desired position. Usually, these deviations can be tolerated only to fractions of the smallest line width to be printed, e.g., for 180 nm patterns the tolerance is .apprxeq.20 nm.
For the purpose of making and assembling such a lithography apparatus, the above mentioned tolerances are strictly confined, for example for electrodes with a diameter of 1 m or more to a roundness of better than .+-.2 .mu.m for each lens electrode, to positioning accuracy of .+-.2 m and to 30 .mu.rad maximum tilt. World wide, such a making and assembling precision is met by only very few sites. Thus, the state of the art manufacturing costs are likely to render apparatus of this type unfavorable for industrial application.
For industrial semiconductor fabrication, it is advantageous to use the so-called "Mix and Match" mode, in which the less demanding parts with regard to line width and resolution of the chip are made by well-established optical lithography, whereas the layers with high performance requirements are produced by, e.g., ion beam lithography, in particular projection lithography. In this "Mix and Match" mode, it is necessary to adapt the shape of the pattern imaged by ion projection lithography to the shape achieved by optical lithography. That means that a tool is needed by which the imaging parameters may be varied and controlled.
In U.S. Pat. No. 4,859,857, which is incorporated by reference, a projection lithography apparatus is disclosed, having a multipole correction element, which is located adjacent to the mask in order to control the shape of the pattern transferred to the substrate. Said correction element comprises electrostatic and/or magnetic multipoles, i.e., a number of, e.g., cylindrical rods and current wires forming magnetic poles arranged around the particle beam. This arrangement is capable of successfully correcting misplacements in the mask pattern which may arise, e.g., from local distortions of the mask. The same arrangement can be used to adjust the image distortion at the substrate level to achieve overlay in a mix and match mode.
In the article entitled "Lens and Deflector Design for Microcolumns," M. G. R. Thompson and T. H. P. Chang in Journal of Vacuum Science Technologies, Vol. B13, No. 6:2445-2449 (November/December 1995), discuss the problem of alignment tolerances in proposed microcolumn systems using electrostatic lenses and deflectors for focusing and scanning electron probes. In optimizing a microcolumn system of this type, the article proposes an integration of the scanning deflectors into the lens by segmenting one or more of the lens electrodes and superimposing the deflection voltages to the lens electrode voltage in order to achieve a better deflection performance.
What is needed is a novel concept for making electrostatic lenses that are capable of relaxing the currently defined tolerance constraints. In this context, it is highly desirable to manufacture and assemble lens components by affordable precision machinery widely available in the art. Further, a feature of the present invention is to provide a lithography apparatus making use of the novel lens concept in order to enable manufacturing, assembling and disassembling of lens columns at a readily affordable cost.